Typically, two-dimensional, 2-D, free-standing crystals exhibit properties that differ from those of their three-dimensional, 3-D, counterparts. Currently, however, there are relatively few materials which can be described as 2-D, atomically-scaled layered solids. Clearly the most studied freestanding 2-D material is graphene, but other materials include hexagonal BN, certain transition metal oxides, hydroxides, and silicates, including clays, S2N, MoS2 and WS2 are also known. Currently, the number of non-oxide materials that have been exfoliated is limited to two fairly small groups, viz. hexagonal, van der Waals bonded structures (e.g. graphene and BN) and layered metal chalcogenides (e.g. MoS2, WS2, etc.).
Although graphene has attracted more attention than all other 2-D materials together, its simple chemistry and the weak van der Waals bonding between layers in multi-layer structures limit its use. Given the properties of graphene for applications ranging from composite reinforcement to electronics, there is interest in other new materials which may also be described as 2-D, atomically-scaled layered solids.